FIG. 1 shows an example configuration of a circuit for one pixel (pixel circuit) in an active organic EL display apparatus. With the source connected to a power line PVdd, a P channel drive TFT 1 has the drain connected to the anode of an organic EL element 3, whereas the organic EL element 3 has its cathode connected to a cathode power supply CV. The source of an N channel selection TFT 2 is connected to the gate of the drive TFT 1, whereas the selection TFT 2 has its drain connected to a data line Data with the gate connected to a gate line Gate. One end of a holding capacitor C is connected to the gate of the drive TFT 1, whereas the other end of the capacitor C is connected to a capacitor power supply line Vsc.
Therefore, a data signal is stored in the holding capacitor C by pulling up the horizontally extending gate line to H level, turning on the selection TFT 2 and under these conditions applying a data signal having a voltage appropriate to the display brightness to the vertically extending data line Data. This allows the drive TFT 1 to supply the organic EL element 3 with a drive current appropriate to the data signal, resulting in the organic EL element 3 emitting light.
In this case, light emission amount and current of the organic EL element are roughly proportional to each other. Normally, a voltage (Vth) is applied between the gate of the drive TFT 1 and PVdd such that a drain current begins to flow near the image black level. As for image signal amplitude, an amplitude is applied such that a given brightness is available near the white level.
FIG. 2 shows the relationship between input signal voltage of the drive TFT 1 (gate-source voltage Vgs=difference between the voltage of the data line Data and the power supply PVdd) and a current icv (corresponding to brightness) flowing through the organic EL element 3. It is possible to perform proper gradation control in the organic EL element 3 by determining a data signal such that Vth is applied as black level voltage and Va as white level voltage.
In this case, the organic EL display apparatus is configured by a display panel in which a number of pixels are arranged in a matrix form. For this reason, Vth varies from one pixel to another due to manufacturing problems, occasionally resulting in a pixel-to-pixel variation of optimal black level in a single display panel. This gives rise to an uneven light emission amount relative to data the signal (input voltage), resulting in uneven brightness.
For this reason, correction is proposed in which each pixel brightness is measured and the black level voltage is corrected for each of all pixels according to correction data stored in memory (Japanese Patent Application Laid-Open Publication No. 1999-282420).
As shown in FIG. 3, however, the pixel drive TFT may present a variation of not only Vth (ΔVth) but also V-I slope (gm). That is, a mean pixel characteristic in line n (b) differs from a mean characteristic of all pixels (a) not only in Vth by ΔVth but also in slope of the drive current icv with respect to the input voltage, with the difference between the characteristics (a) and (b) not being uniform at input voltages Va1, Va2 and Va3. In such a case, correction of Vth alone does not allow for sufficient correction.